[MD-Settings A11y] Set alt text for expandable buttons.

chrome/browser/resources/settings/internet_page/internet_detail_page.html

 <link rel="import" href="chrome://resources/cr_elements/cr_expand_button/cr_expand_button.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_icon.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_onc_types.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_network_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_network_indicator.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-collapse/iron-collapse.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
@@ skipping 166 matching lines @@
               property-dict="[[networkProperties]]">
           </network-property-list>
         </div>
       </template>
     </div>
 
     <!-- Advanced toggle. -->
     <div class="settings-box" actionable on-tap="toggleAdvancedExpanded_"
         hidden$="[[!showAdvanced_(networkProperties)]]">
       <div class="advanced flex">$i18n{networkSectionAdvanced}</div>
-      <cr-expand-button id="expandButton" expanded="{{advancedExpanded_}}">
+      <cr-expand-button id="expandButton" expanded="{{advancedExpanded_}}"
+          alt="$i18n{networkSectionAdvancedExpandA11yLabel}">
       </cr-expand-button>
     </div>
 
     <!-- Advanced section -->
     <iron-collapse opened="[[advancedExpanded_]]">
       <div class="layout vertical indented">
         <div class="settings-box single-column stretch"
             hidden$="[[!hasAdvancedOrDeviceFields_(networkProperties)]]">
           <!-- Advanced properties -->
           <network-property-list
@@ skipping 36 matching lines @@
           <span class="subtitle">$i18n{networkSectionProxy}</span>
           <network-proxy editable on-proxy-change="onProxyChange_"
               network-properties="[[networkProperties]]">
           </network-proxy>
         </template>
       </div>
     </iron-collapse>
   </template>
   <script src="internet_detail_page.js"></script>
 </dom-module>